The use of surface to borehole electromagnetic (EM) methods has a long history in the mining industry. In general, such techniques use a man-made EM source located at the surface of the Earth, with measurements made of various EM components downhole. Compared to surface-only survey methods, surface to borehole techniques offer improved resolution at depth in the region of the borehole. To date, known surface-to-borehole methods generally have described measurements made after a borehole has already been drilled, rather than during the drilling process, and are generally directed to reservoir characterization post-drilling for applications such as fluid front monitoring and production monitoring and control. A borehole-to-surface survey geometry, such as that described in U.S. patent application Ser. No. 12/603,053, filed Oct. 21, 2009 incorporated herein by reference in its entirety, is also known for measurements made after a borehole has been drilled.
For example, WO2005085909 entitled “Combined surface and borehole electromagnetic measurement system and method for determining formation fluid properties” to Strack describes surface-to-borehole survey measurements post-drilling for monitoring the movement of fluid in the reservoir. U.S. Pat. No. 5,886,255 to Aronstam describes seismic borehole sensors for production monitoring when drilling is complete. U.S. Pat. No. 5,597,042 and U.S. Pat. No. 5,662,165, both to Tubel et al., describe permanently installed sensors of various types for reservoir monitoring and production control when drilling is complete.
It is highly desirable to detect formation characteristics ahead of and/or about the drilling apparatus while drilling is proceeding, so as to avoid problems arising from entering an over-pressurized zone, problems arising from crossing faults and bed boundaries, and the like. However, known while drilling look-ahead and/or look-around do not detect formation anomalies at a sufficient time and distance ahead of the bit or BHA with an acceptable spatial resolution and depth of investigation. Additionally, the complex inversion methods described in related art for reservoir characterization, fluid monitoring and production control are not suitable for the while-drilling applications proposed here, given that the computational complexity and time required to complete them would not enable an answer in time to avoid the problems described above.
One specific pressing problem experienced during drilling of a petroleum well is detecting the base of salt bodies prior to exiting. Sediment below a salt body can be over-pressured, a condition that leads to blow outs and other dangerous situations if the drill rig and controls are not set up to accommodate excess pressure. As such, accurately and quickly detecting salt bodies well ahead of the drill bit during drilling is highly valued.